Some weight painted faces not effected?
This kind of issues arise when you try to smooth the weights of the main area bone (in your case the neck bone) but you forget to smooth the neighbour bones (probably the head bone). 

In particular if two adiacent vertices have the same minimum weight (let's say 0.001) to the head bone, but one belongs to the head bone only while the other has a tiny amount of weight from the neck bone also, their deformation will be completely different, no matter if they look the same pale blue in weight paint mode: if a vertex belong to a single bone/vertex group only, it will follow the movements of that bone exactly, even if its weight is very tiny.

I'm pretty sure that if you highlight the head weights, you will see that they end exactly where you have the sharp corner, and this is where you'll have to use the smooth brush until you reach the desired deformations.